The present invention relates to automotive crash absorbing components and more particularly to structural members that connect the bumper to an automotive frame tuned to control energy absorption.
A variety of energy absorbing components have been used in mounting automotive bumpers. Some components incorporate springs, shear pins, hydraulic fluids and gels and explosive charges which react to compressive loads to dissipate energy. While these energy absorbing devices have performed adequately, they are often bulky and incapable of absorbing the energy of an impact in a controlled, stabilized fashion. Other devices utilize sacrificial structural components that are designed to collapse under the loading of a vehicle collision. Most of these devices are lighter and more cost effective than springs or other reusable devices. Some sacrificial structural components utilize a tubular structure that has a generally constant cross-section. Convolutions or holes are incorporated into the tubular devices to encourage a controlled collapse.
One objective of a controlled dynamic crushing mode for these devices is to prevent a plastic hinging and bending of the device. In a bending mode, the device can fold, allowing the bumper to approach the frame with relatively little energy absorption. In this type of undesired bending mode, residual energy is translated to the frame of the vehicle resulting in a higher loading on the vehicle and its passengers.
Another objective is to control the impact energy in as constant a fashion as possible to limit potential high impact loading on the vehicle. Convolutions can sustain the crush sequence in a desired bumper to frame direction to provide for maximum energy absorption. Out-of-sequence crushing results in less energy absorption by the device and can promote an undesired plastic hinge bending. Traditionally, rail tip designs have had rectangular cross-sections with the axis of maximum moment of inertia aligned with a horizontal Y axis. Under normal vehicle operating conditions, bending about axes parallel to the Y axis is a predominant concern. Rectangular cross-sections also provide large flat surfaces ideal for bracketry and body mount attachments. The rectangular cross-section, however, is not an ideal design for dynamic loading because it""s resistance to bending is essentially unilateral. During axial crush, it is susceptible to inboard or outboard bending about the axis of minimum moment of inertia.
In the present invention, a rail tip is provided which interconnects the automotive frame and bumper. The rail tip absorbs kinetic energy during impact by crushing sequentially bumper to frame in a natural mode without reliance upon convolutions and/or holes to sustain the crush sequence. The design utilizes stability theory and empirically developed data to offer greater reliability, higher energy absorbing efficiency and greater stability against plastic hinging and bending. The rail tip of the present invention is tuned by careful selection of the octagonal circumference, wall thickness, length wise taper and material properties to crush under a design load and absorb a predetermined amount of kinetic energy over the crushable length. The octagonal-shaped rail tip is hydro formed from a cylindrical tube rather than constructed in the traditional method of two halves stamped, overlapped and welded. Fabricated in this manner, it is less likely to have flaws in form, fit and welding that could adversely affect axial crush and induce plastic hinge bending or out of sequence folding during crush. The octagonal cross-section shape tapers from a perimeter approximately 13 inches at the bumper end to approximately 15 inches at the frame end to better resist side loads during crushing. The wall thickness also tapers to maintain an advantageous ratio of wall thickness to perimeter. Together these tapers promote sequential bumper to frame axial crush. A buckling initiator can also be provided to reduce the peak buckling load while initiating bumper to frame crush. The front end sheet metal body mount attaches through the center line of the rail tip rather than hanging from one side. This reduces the magnitude of side loading introduced by the body into the mid-rail tip during crush.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limited the scope of the invention.